1. Field of the Invention
The present invention relates to an optical waveguide device having an optical waveguide of a ferroelectric thin film and a thin film lens for controlling a laser beam incident to the optical waveguide, and it has an application use in general optical integrated devices including, for example, optical deflection devices used for laser printers, digital copying machines and facsimile units optical switches and optical modulation devices used for optical communication and optical computers, and pick-up devices used for optical discs.
2. Prior Art
For disposing a deflection device, a switching or modulation device for a laser beam in an optical waveguide to constitute an optical integrated circuit device, it is important to form an electrode for deflection, switching or modulation to the optical waveguide, as well as use a waveguide lens for collimating a laser beam incident to the optical waveguide, and deflecting, switching or modulating and then collecting the laser beam. Particularly, the waveguide lens is an indispensable element in an optical deflection device using a planar waveguide.
As an optical deflection device using a planar optical guide, an optical deflection device utilizing an acousto-optical effect is disclosed, for example, in C. S. Tsai and P. Le, Appl. Phys. Lett. vol. 60 (1992) 431 (hereinafter referred to as literature (1)), in which a comb electrode for exciting surface elastic waves is disposed on the surface of an optical waveguide for Bragg diffraction of an optical beam in the optical waveguide and deflection is conducted by sweeping the frequency of the surface elastic waves. Further, in a prism optical deflection device using an electro-optical effect, a prismatic electrode is disposed on the surface of the optical waveguide, and the refractive index of the optical waveguide below the electrode is changed by the application of a voltage to deflect the optical beam in the optical waveguide as disclosed, for example, in Q. Chen, et al., J. Lightwave Tech. vol. 12 (1994) 1401 (hereinafter referred to as literature (2)).
As the material for the planar waveguide, glass such as quartz, oxide ferroelectrics such as LiNbO.sub.3, polymers such as PMMA or GaAs series compound semiconductors are used. Among them, the ferroelectric oxide materials such as LiNbO.sub.3 have good acousto-optic effect or electro-optical effect, and made of devices manufactured actually utilizing the effects described above are composed of LiNbO.sub.3, and an optical deflection device utilizing the acousto-optical effect and the prism deflector utilizing the electro-optical effect are shown in the literatures (1) and (2), respectively.
In addition to LiNbO.sub.3, there are various ferroelectrics such as BaTiO.sub.3, PbTiO.sub.3, Pb.sub.1-x La.sub.x (Zr.sub.y Ti.sub.1-y).sub.1-x/4 O.sub.3 (PZT, PLT, PLZT depending on the values for x and y), Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3, KNbO.sub.3, LiTaO.sub.3, Sr.sub.x Ba.sub.1-x Nb.sub.2 O.sub.6, Pb.sub.x Ba.sub.1-x Nb.sub.2 O.sub.6, Bi.sub.4 To.sub.3 O.sub.12, Pb.sub.2 KNb.sub.5 O.sub.15 and K.sub.3 Li.sub.2 Nb.sub.5 O.sub.15 and most of the materials described above have more satisfactory characteristics than LiNbO.sub.3. Particularly, Pb.sub.1-x La.sub.x (Zr.sub.y Ti.sub.1-y).sub.1-x/4 O.sub.3 has been known as a material to provide an electro-optical coefficient much higher than that of LiNbO.sub.3. While the LiNbO.sub.3 single crystal has an electro-optical coefficient of 30.9 pm/V, PLZT (8/65/35:.times.=8%, y=65%, 1-y=35%) ceramics can provide an electro-optical coefficient of 612 pm/V.
While there are a lot of ferroelectrics having more satisfactory characteristics than LiNbO.sub.3, LiNbO.sub.3 is used in most of actually manufactured devices. This is chiefly because a thin film had to be grown epitaxially excepting for LiNbO.sub.3, for which the optical waveguide technique was established based on the single crystal growing technique and the Ti diffusion and proton exchange to a wafer thereof, so that a thin film optical waveguide could not be manufactured with a quality at a practical level by existent vapor phase growing, and because there was no available technique for manufacturing a waveguide lens even if the thin film optical waveguide per se could be manufactured. On the contrary, the present inventors have already invented a method of manufacturing a thin film optical waveguide with a quality at a practical level by the solid phase epitaxial growing technique, with respect to the manufacture of the thin film optical waveguide with a quality at a practical level, disclosed the relevant invention in Japanese Published Unexamined Patent Application No. Hei 7-78508 and overcome the problem that the optical waveguide with the quality at the practical level could not be manufactured.
Meanwhile, the waveguide lens can be classified into five systems, namely, Mode index, Luneburg, Geodesic, Fresnel and Grating system, which are shown in the literature (Optical Integrated Circuit, by Nishihara, Haruna and Narahara, published from Ohmsha (1993) pp. 291-304).
In the mode index lens, regions each having an effective refractive index which is different stepwise are formed in a waveguide, and known methods of forming the difference of the effective refractive index include (1) a method of utilizing the difference of the thickness in the lens shape, (2) a method of coating a lens-shaped high refractive index layer, (3) a method of burying a lens-shaped high refractive index layer, (4) a method of conducting diffusion or ion exchange of other elements in a lens shape in the waveguide and (5) a method of patterning a portion of the waveguide into a lens shape and then refill the pattern into a flat shape by other materials. Since the planar process can be utilized for the lens, it can provide high mass productivity and even a non-spherical shape can also be manufactured easily.
However, in the method (1) of utilizing the difference of the thickness, the method (2) of coating the high refractive index layer and the method (3) of burying the high refractive index layer, coupling loss tends to be increased in principle, due to the presence of the step, by scattering at a lens boundary, lowering of overlap integral value and conversion of mode. The method (4) of conducting diffusion or ion exchange of other elements in the waveguide and the method (5) of patterning a portion of the waveguide and then refilling by use of other materials are suitable to the manufacture of a satisfactory waveguide lens.
However, a method of conducting diffusion or ion exchange of other element is not available for the material other than LiNbO.sub.3, particularly, for Pb.sub.1-x La.sub.x (Zr.sub.y Ti.sub.1-y).sub.1-x/4 O.sub.3. Further, Japanese Published Unexamined Patent Application No. Hei 3-291604, etc. show a method of refilling an SiON optical waveguide made of glass material disposed on quartz glass with other materials by a lift off method after etching. However, there is no available method for selectively applying etching without giving surface roughness which would lead to the scattering loss in the optical waveguide of single crystal epitaxial ferroelectric thin film and without giving damages to a substrate composed of the same kind of oxide as the thin film optical waveguide. Therefore, manufacture of a mode index lens in the optical waveguide of epitaxial ferroelectric thin film is not reported so far.
The Luneburg lens is a soft of the mode index system (2) in which a circular high refractive index film having gradually varying thickness is disposed on a waveguide. In principle, it has a feature capable of forming a non-aberration lens, but it is not industrially suitable since the shape with gradually varying film thickness cannot be manufactured easily at a good reproducibility.
The Geodesic lens is of a system in which a cup-shaped concave portion is formed to a substrate, on which a waveguide is disposed and this is only one system capable of focusing images with no aberration also to multi-mode propagation light. However, it is difficult to machine the concave portion at high accuracy and good productivity on the substrate and this is not industrially suitable, either.
Both of the Fresnel lens and the Grating lens are of a system utilizing diffraction, but they are identical with the mode index lens in that regions having an effective refractive index which is different stepwise are formed in the waveguide. They have a feature capable of being formed by a planar process and a feature in that the optical system is compact. On the other hand, since the pattern is fine, it has to be manufactured by electron bean (EB) lithography which is poor in the product at present. In addition, it may result in straight forward light in addition to condensed light to cause cross-talk, and characteristics are sometimes changed greatly depending on the change of the incident angle or incident wavelength.
As described above, while various waveguide lens systems and manufacturing methods thereof are known, there are no waveguide lens system having satisfactory characteristics as the lens and good productivity and applicable to optical waveguides of epitaxial ferroelectric thin film, as well as manufacturing methods thereof, leaving a problem of incapable of forming waveguide lens to optical waveguides of ferroelectric thin film having high characteristics thereby attaining highly functional optical integrated devices.